Multi-layer Optical Disc And System

ABSTRACT

A multi-layer optical disc comprises multiple storage and reference layers. The storage layers each have a distinct distance from its reference layer. Beam portions of a read-out beam are reflected by the storage and reference layers respectively. Interference among the reflected beam portions is tuned to retrieve stored information.

CROSS REFERENCE TO RELATED APPLICATION

This is a division of U.S. Ser. No. 10/367,510, filed Feb. 14, 2003.

BACKGROUND

1. Field of Invention

This invention is related to optical data storage media and systems, particularly to multi-layer optical data storage media and optical storage systems using such media.

2. Description of Prior Art

Most optical discs, including a compact disc (CD) and a digital versatile disc (DVD), have a single storage layer for storing information. Some discs contain double storage layers to increase the capacity. To read double storage layers, an objective lens is moved between two positions, which in turn moves the focal position of a read-out beam such that the beam is focused onto each layer respectively. Similar read-out methods are also used for more than two storage layers. Obviously, the maximum allowable number of storage layers in a multi-layer disc is determined by the spacing between two adjacent storage layers and the working distance of the objective lens. But the spacing has to be large enough to avoid crosstalk between neighboring storage layers. Depending upon each individual system, the spacing ranges from 30 to 80 micrometers.

In order to place storage layers more closely in an optical disc, other methods have been proposed to read a layer without severe crosstalk from its neighboring layers. One of them employs techniques of optical coherence tomography (OCT), which is an emerging technology and has great potentials in biomedical applications.

An OCT system has a low-coherence light source which emits a beam with a relatively short coherence length. Currently at the heart of OCT is an amplitude division interferometer, usually a Michelson interferometer. An OCT system splits a beam into two beams by a beam splitter. One beam propagates to a reference reflector along a reference optical path, and the other beam to a sample medium along a sample optical path. The beams reflected by the reference reflector and the sample medium are then recombined by the beam splitter.

Due to the nature of low coherence, the combined beams interfere with each other only when their optical path length difference is within the beam's coherence length. The interference intensity and pattern contrast reach a maximum when the two path lengths are matched. For highly scattering sample media, various sample paths yield different optical path lengths, depending upon where a beam is reflected inside the media. Since a reference optical path length can be adjusted to match a sample optical path length, tuning the reference path length results in interference between the reference beam and a sample beam which is reflected from a layer at a depth inside the media. The interference intensity and patterns are related to the layer's optical properties, such as refractive index, birefringence, scattering coefficient, etc. Coherence length of the beam determines measurement resolution along the beam propagation direction. The shorter the coherence length is, the higher the measurement resolution. By combing the low coherence interference technique with a laterally scanning mechanism, a three-dimensional image can be constructed.

Naturally, an OCT scheme can be used to read multiple storage layers in an optical disc. The storage layers are partially reflective and partially transmissive, and distributed in three dimensions. Since optical path length is of the product of a path length and refractive index along the path, the minimum distance between adjacent layers is of half the beam's coherence length divided by the refractive index. For a broadband light source, its coherence length can be in the order of 1 micrometer. Thus an optical disc using OCT techniques can have much smaller layer spacing and hold much more storage layers than a conventional optical disc.

There are several references using OCT methods for a multi-layer optical disc. See, for example, U.S. Pat. No. 5,883,875 (1999) to Coufal, et al. and U.S. Pat. No. 6,072,765 (2000) to Rolland, et al. As a result, the multi-layer medium only contains storage layers, while the reference reflector is built within the OCT system. Since read-out results depend upon an optical path length to a storage layer, medium vibration causes a change of the optical path length and brings measurement errors. The setup inherits drawbacks of a current OCT: Sensitivity to sample vibration and a bulky structure due to separate sample and reference paths.

Accordingly, there is a need for a multi-layer optical disc which contains more storage layers, and a multi-layer optical disc system which is able to read such a disc.

OBJECTS AND ADVANTAGES

Accordingly, several main objects and advantages of the present invention are:

-   -   a). to provide an improved multi-layer optical disc;     -   b). to provide such an optical disc which comprises multiple         storage and reference layers, where each storage layer has a         distinct distance from its corresponding reference layer;     -   c). to provide an improved multi-layer optical disc system;     -   d). to provide such a system which employs a relatively simple         and compact interference structure; and     -   e). to provide such a system which retrieves information using         vibration-insensitive interference between beam portions         reflected by the storage and reference layers.

Further objects and advantages will become apparent from a consideration of the drawings and ensuing description.

SUMMARY

In accordance with the present invention, a multi-layer optical disc and an optical disc system are constructed. The multi-layer optical disc comprises multiple storage and reference layers, where each storage layer and its reference layer have a distinct spacing between them.

The optical storage system retrieves data using adjustable interference between beam portions reflected by the storage and reference layers. The beam portions are split by wavefront division in a relatively simple and compact structure. Since the beam portions are reflected by the layers within the disc, the interference result is insensitive to disc vibration.

Abbreviations

AR Anti-reflection

CD Compact Disc

DVD Digital Versatile Disc

HR High Reflection

OCT Optical Coherence Tomography

PR Partial Reflection

DRAWING FIGURES

FIG. 1-A is a schematic diagram showing a prior-art double layer optical disc and a read-out method.

FIG. 1-B is schematic diagram showing a prior-art multi-layer optical disc system using an OCT scheme.

FIG. 2 is a schematic diagram illustrating an interferometer for measurement using wavefront division according to the invention.

FIG. 3 is a schematic diagram illustrating an embodiment of a multi-layer optical disc system according to the invention.

FIG. 4 is a schematic cross-sectional view illustrating an embodiment of a multi-layer optical storage arrangement according to the invention.

FIGS. 5 and 6 are schematic cross-sectional views illustrating embodiments of a multi-disc optical data storage arrangement according to the invention.

REFERENCE NUMERALS IN DRAWINGS

12 collimated beam 14 modulator element 16 modulator element 17 spatial phase modulator 18 beam portion 20 beam portion 21 sample 23 sample surface 26 HR reflector 30 lens system 35 beam portion 36 beam portion 50 detector 52 lens system 54 beam splitter 63 detector 71 light source 72 reflective reference layer 74 PR storage layer 76 PR storage layer 78 PR storage layer 86 PR storage layer 91 optical disc 93 optical disc 95 PR reference layer 96 optical disc 97 optical disc 98 PR reference layer 99 PR storage layer 104 read-out beam 106 read-out beam 108 optical disc 110 storage layer 112 storage layer 116 optical disc 118 multi-layers 120 beam splitter 130 optical disc 132 multi-layers

DETAILED DESCRIPTION—FIGS. 1-A AND 1-B—PRIOR-ART MULTI-LAYER DISC AND SYSTEM

FIG. 1-A shows schematically a read-out beam reads a double layer optical disc 108 in a prior-art optical disc system. Disc 108 contains two storage layers 110 and 112. A read-out beam 104 is focused on layer 110 initially. To read layer 112, which is beneath layer 110, an objective lens (not shown in FIG. 1-A) of the system is moved closer to the disc so that the read-out beam's focal position penetrates deeper in the disc, as illustrated by another read-out beam 106. Since the beams each interact with both layers simultaneously, the spacing between the layers has to be larger enough to avoid crosstalk.

FIG. 1-B shows schematically an OCT scheme is employed to read a multi-layer disc. The OCT is basically a Michelson interferometer comprising a low-coherence light source 71, a beam splitter 54, an adjustable reference reflector 26, a detector 63, and an optical disc 116 having multiple storage layers 118. A beam from source 71 is spit into a reference beam and a sample beam by splitter 54 through amplitude division. The reference beam is transmitted to reflector 26, reflected by the reflector, and then propagates to detector 63 after passing through splitter 54. The sample beam impinges onto disc 116 and is reflected by multiple layers 118. Then the reflected sample beam is reflected by splitter 54 and combined with the reference beam. The two beams generate a low-coherence interference which is detected by detector 63. Since the reflected sample beam contains multiple portions caused by multiple reflections of the storage layers, reflector 26 is adjusted such that the optical path length of the reference beam matches that of a sample beam portion which is reflected by a specific storage layer. The minimum storage layer spacing is of half the coherence length of light source 71 divided by the refractive index. However as discussed in the background section, the system is bulky due to separate sample and reference paths and because any disc movement in a direction the read-out beam travels affects the sample beam path length, the system is also sensitive to vibration, which makes it difficult for practical use.

FIGS. 2 and 3—Novel Interferometer and Optical Disc System

FIG. 2 illustrates schematically an interferometer for optical measurement. A collimated beam contains beam portions 18 and 20, which are used as sample and reference beams. The beam portions impinge onto a surface 23 of a sample 21 after being transmitted through a beam splitter 120. Beam portions 18 and 20 are reflected by surface 23, and then by splitter 120. Finally, the beam portions interfere with each other when they are mixed and focused onto a detector 50 by a lens system 52. The interference can be tuned by adjusting phase difference between the beam portions using a spatial phase modulator.

Compared with the setup of FIG. 1-B, the sample and reference beams in the interferometer here are side-by-side, thus the system becomes simpler and more compact. Furthermore, the interference result is insensitive to sample vibration, because the two beam portions experience the same path length change during the vibration. The interference results may be used to profile surface 23.

The interferometer configuration of FIG. 2 can be modified for use in a multi-layer optical disc system, as shown schematically in FIG. 3. A collimated beam 12 enters a spatial phase modulator 17 and is processed by modulator elements 14 and 16. When beam 12 leaves the modulator, it becomes beam portions 18 and 20 with a tunable phase difference. The beam portions are transmitted through a beam splitter 120 and focused on a region of multiple layers 132 of an optical disc 130 by a lens system 30. Layers 132 comprise multiple storage and reference layers, instead of storage layers only in a conventional optical disc. Next, the beam portion are reflected by layers 132 and collimated by lens system 30. Then, as in FIG. 2, the reflected beam portions are reflected again by splitter 120 and focused onto detector 50, where interference signals are observed. The reflected beam portions contain multiple waves whose phase is determined by modulator 17 and layers 132. A wave reflected by the reference layer becomes a reference beam, while a wave reflected by a storage layer becomes a sample beam. By tuning modulator 17, the multi-wave interference is adjusted accordingly. When beam 12 is of low-coherence, low-coherence interference can be used to single out a sample beam and detect the reflectivity of the corresponding storage layer as illustrated in the following paragraphs.

The system of FIG. 3 is simpler and more compact than systems using the current OCT structure, because the sample and reference beams are not separate. In addition, the disc comprises both sample and reference reflectors, thus path length difference between sample and reference beams is insensitive to disc vibration, so is the read-out result.

FIGS. 4-6—Multi-Layer Optical Storage Structures

A schematic cross-sectional view in FIG. 4 illustrates an embodiment of structure and read-out method for a multi-layer optical disc. The multi-layer optical disc contains two regions. A first region has a single partial reflection (PR) or high reflection (HR) reference layer 72 as a reference reflector. A second region has PR storage layers 74, 76, and 78 as storage reflectors. Surrounding the reference and storage layers are low-loss transmissive materials. Stored data are represented by either a partial reflection or a relatively low reflection of the storage layer. Read-out beam portions 35 and 36 are created and tuned by a spatial phase modulator (not shown in FIG. 4), and are aligned to the two regions respectively. Portion 35 impinges onto layer 72, while portion 36 impinges onto the storage layers.

For portion 35, it has only one reference optical path involving one reflection from layer 72. But due to three storage layers, beam portion 36 has three storage optical paths containing a single reflection, and various storage optical paths containing multiple reflections. One storage path of portion 36 has a route from the modulator to layer 74, to layer 76, to layer 74, to layer 76, to layer 74, and finally to a detector. The spacing between adjacent storage layers should be equal or larger than half the beam's coherence length divided by the refractive index. For read-out purpose, only three storage paths involving a single reflection are needed. The three paths each have a respective optical path length. By tuning the spatial phase modulator, the reference optical path length can be adjusted to match any of the storage optical path lengths. Therefore, reflectivity of the three storage layers can be detected.

A schematic cross-sectional view in FIG. 5 illustrates an embodiment of a multi-disc optical data storage configuration. Again beam portions 35 and 36 work as reference and sample beams, respectively. Optical discs 91 and 93 each contain a PR reference layer 95 and a PR storage layer 86. Surrounding the reference and storage layers are low-loss transmissive materials. The discs are stacked and their reference layers and storage layers are aligned in a direction perpendicular to the layers. Layer 95 functions as a reference reflector for storage layer 86 in the same disc. Layer 86 stores information by having different reflectivity values. Each disc has a distinct spacing between layers 86 and 95 in a direction perpendicular to the layers or portions 35 and 36 travel along, which is equal or larger than half the beam's coherence length divided by the refractive index. Distance between layers 95 in the discs should be large enough to avoid any unwanted interference.

To read out data, beam portions 35 and 36 are transmitted to impinge onto the discs. Each beam portion impinges on two layers which are in separate discs. Consider reflected beams with only one reflection. There are total four reflected beams, among which two are reference beams bounced by reference layer 95 and the other two are sample beams generated by sample layer 86. In other words, discs 91 and 93 each create a sample and a reference beam. The sample and reference beams have a path length difference which is affected by the spacing between the layers. Since the phase of portions 35 and 36 is tunable through a spatial phase modulator (not shown in FIG. 5), the modulator can be used to compensate the phase difference caused by the spacing and match the sample optical path length to the reference path length for a disc. To avoid matching two pairs of path length at the same time, the spacing difference between layers 86 and 95 in the discs should be equal or larger than half the beam's coherence length divided by the refractive index. Therefore, although there are four reflected beams from two discs in FIG. 5, it is possible to produce a low-coherence interference between two beams from one disc. Therefore as in FIG. 4, interference between the reflected beams can be used to measure the reflectivity of storage layer in the disc.

FIG. 6 illustrates another embodiment of a multi-disc optical data storage configuration through a schematic cross-sectional view. In the embodiment, a PR storage layer 99 overlaps a PR reference layer 98 in discs 96 and 97. Surrounding the reference and storage layers are low-loss transmissive materials. The two discs are stacked together. Beam portions 35 and 36 each impinge onto the four PR layers in two discs. Again the spacing between a reference and a storage layer in each disc is distinct, equal or larger than half the beam's coherence length divided by the refractive index, and is used to distinguish storage layers in the discs. And thickness of each disc is larger than the beam's coherent length.

The overlapping layers in FIG. 6 mean that the beam portions have the same optical path lengths inside the storage medium; in other words, separation of beam portions becomes not necessary in the medium. Once again, consider reflected beams having one reflection only, since a reflected beam bounced between layers has at least three reflections, which reduced its intensity greatly. When portions 35 and 36 are reflected by the reflectors of the discs, eight reflected beams are created. The phase of each reflected beam is determined by a path through which it travels in the storage medium and a spatial phase modulator (not shown in FIG. 6) which tunes the phase of portions 35 and 36. There are three spacings between the four layers in FIG. 6. To avoid unwanted interference, each spacing has to be equal to or larger than a value, which we call value A, as discussed before. Value A is of half the beam's coherence length divided by the refractive index. Assume the spacing between 98 and 99 layers in disc 96 is of value A, between 98 and 99 in disc 97 twice value A, and between 98 in disc 96 and 99 in disc 97 at least three times of value A. Then when the modulator adjusts the phase of portions 35 and 36, and makes the optical path length of a beam reflected by layer 98 of disc 96 match that of a beam reflected by layer 99 of the same disc, it creates the only interference, since the path length difference between any other two beams is larger than the coherence length. The only interference happens again between beams reflected by layers 98 and 99 of disc 97, when their path lengths are matched. Therefore as before, the low-coherence interference method can be used to select two beams which are reflected by a reference and a storage layer in the same disc, and reveal the storage layer reflectivity.

CONCLUSION, RAMIFICATIONS, AND SCOPE

Accordingly, the reader will see that a multi-layer optical disc has multiple storage and reference layers. Each storage layer has a distinct distance from its corresponding reference layer. And a multi-layer optical storage system retrieves data from the multi-layer disc using adjustable interference among beam portions which are reflected by the storage and reference layers respectively.

Although the description above contains many specificities, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments. Numerous modifications, alternations, and variations will be obvious to those skilled in the art. For example, a single disc containing multiple reference and storage layers can replace the multi-disc structures in FIG. 5 or 6. To replace the scheme in FIG. 4, storage layers may be arranged in multiple discs and have a common reference reflector attached to them. A beam may be divided into portions of any number with any geometrical shapes by wavefront-division; for example, a beam may be divided into a central circular portion and several outer ring-shaped portions. The intensity ratio of one portion to another can be of any value depending upon the interference effect between them. For example, if a portion is intended to be reflected by a reflector of low reflectivity, this portion should have a larger intensity in order to improve contrast of interference patterns. Lastly in FIG. 5, the reference layers in two discs may be misaligned to the storage layers based on the schemes shown in FIG. 6.

Therefore the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given. 

1. A multi-layer optical data storage medium comprising: 1) a plurality of storage layers as storage reflectors for storing data; and 2) at least one reference layer as a reference reflector; 3) said storage and reference layers being arranged such that there is a distinct optical path length between said reference reflector and each of said storage reflectors in a direction a read-out beam is transmitted.
 2. The storage medium according to claim 1 wherein at least one of said storage layer is disposed in a discrete storage unit.
 3. The storage medium according to claim 1 wherein said reference layer is disposed in a discrete storage unit.
 4. The storage medium according to claim 1 wherein at least one of said storage reflectors has an adjustable reflectivity.
 5. The storage medium according to claim 1 wherein said storage and reference layers are arranged sharing one substrate.
 6. The storage medium according to claim 1 wherein at least one of said storage layers is disposed to function as said reference layer for another said storage layer.
 7. An optical data storage system comprising: 1) a light source for generating a read-out beam; 2) a multi-layer optical data storage medium comprising a plurality of storage layers for storing data and at least one reference layer, said read-out beam being transmitted to impinge onto said storage medium and reflected by said storage and reference layers respectively, and said storage and reference layers being arranged such that there is a distinct optical path length between said reference reflector and each of said storage reflectors in a direction said read-out beam is transmitted; and 3) a detector for sensing interference among the reflected beams reflected by said storage and reference layers.
 8. The storage system according to claim 7, further including a spatial phase modulator for dividing said read-out beam into a plurality of beam portions by wavefront division and producing phase shift on each said beam portion respectively.
 9. The storage system according to claim 8, further including tuning means for adjusting phase shift of at least one of said beam portions.
 10. The storage system according to claim 7 wherein said light source has relatively low coherence.
 11. The storage system according to claim 7, further including optics means for focusing said read-out beam onto said medium and said reflected beams onto said detector respectively.
 12. A method for retrieving information from an optical data storage medium, comprising: 1) causing a light source to generate a read-out beam; 2) providing a multi-layer optical data storage medium comprising at least one storage layer and at least one reference layer; 3) transmitting a first beam portion of said read-out beam through a first optical path, said first optical path being arranged to connect said source and a detector via said storage layer; 4) transmitting a second beam portion of said read-out beam through a second optical path, said second optical path being arranged to connect said source and said detector via said reference layer; 5) adjusting path length difference between said first and second paths; and 6) sensing interference between said first and second beam portions by said detector.
 13. The method according to claim 12 wherein said light source has relatively low coherence.
 14. The method according to claim 12 wherein said first and second beam portions are generated using methods including wavefront division.
 15. The method according to claim 12, further including focusing said beam portions onto said medium and said detector respectively. 